1. The Field of the Invention
This invention relates to photonic processing and, more particularly, to novel systems and methods for switching and routing photonic signals.
2. Background
Communication signals must be routed through networks from a source to a destination. One form of signal is a photonic signal. Photonic signals may be a series of pulses, or other serial data, and may also be carried in a two dimensional array of pixels, a parallel flow of data.
Fundamental to transmission of any signal over a multiplicity of locations, such as through elements of a network, is routing and switching of signals between potential alternative paths. Photonic signals are electromagnetic waves modulated in some fashion to carry information. Electronic signals are typically routed by electronic switches. Optical signals or photonic signals are typically switched by electro-optical mechanisms. Switches for optical signals or photonic signals do not work well with signals that have variations in state of polarization. For example if polarization is modulated as a mechanism for transmitting information in a photonic signal, the variations in polarization must pass through all elements of a switching or routing network in order to preserve the information embodied in the switched outputs.
Various signal routers have been produced. Particular technologies include micro-mirrors, bubble-jet bubbles, and electronic switches or electro-optical mechanisms. The prior art lacks an effective high speed switching structure for routing a photonic signal through basic switching elements of a network while preserving or stabilizing a state of polarization of the photonic signal. If polarization-sensitive elements exist, including elements that benefit from polarization modulation, then preservation or stabilization of the state of polarization of a photonic signal may be critical.
Switches for routing optical or photonic data have made recent advances. For example, Bergland et al., U.S. Pat. No. 5,317,658, characterize an optical switch as xe2x80x9cPolarization independent.xe2x80x9d According to Bergland, certain specific difficulties associated with polarization dependence are addressed by a switch capable of switching both TE and TM components of a lightwave received by a switch.
However, Bergland et al. state that xe2x80x9cAlthough the polarization-independent switch may switch both the TE and TM components of a light wave in an arbitrary polarized condition, it has the disadvantage of requiring a higher operating voltage than the polarization-dependent photonic switch. Moreover, the level of performance in each individual switch element in the polarization-independent photonic switch is inferior to that of the individual switch elements in the polarization-dependent photonic switch.xe2x80x9d The requirement for using this type of photonic switch inherently limits the usefulness and switching speed.
Another problem is dependence on high birefringence fiber as a required component. Bergland teaches the necessity of using this fiber in order to provide the polarization rotation needed to accommodate his polarization-dependent switches. This requirement introduces specific problems that may not be observed with the older, slower communications equipment, but as bit rates go up, and throughput becomes more important, the signals being switched become more critical. High time division multiplexed (TDM) bit rates require shorter and shorter pulses. These pulses may require special processing both before they are launched into the fiber, and at the various nodes along the way.
A xe2x80x9chigh birefringence fiberxe2x80x9d is a nonlinear optical element. The introduction of such nonlinearity can be a severe detriment in high bit rates systems because it tends to exacerbate the problems of four-wave mixing between the various optical signals, that might otherwise be manageable.
Every extra element inserted into an optical path introduces losses. So, by providing an optical switch that does not require these extra components, losses can be reduced, which becomes increasingly important as the number of switching elements in a switching fabric increases.
The present invention addresses these disadvantages by producing a photonic switch that does not favor one polarization over another. It eliminates the need both for the high birefringence fiber, and even for the need to rotate the polarization in one of the light paths, simplifying the design, reducing losses, and reducing manufacturing costs. The present invention goes on to provide high speed switching even when using low speed components. It introduces the use of optically-controlled, all optical switching, plus a simplified routing arrangement that is compatible with photonic transistor technology and other all optical methods of directing data packets through all optical switching fabrics. Carlsen et al, U.S. Pat. No. 4,474,435 also rotates one of the polarization-separated beams in order to use a xe2x80x9cpolarization sensitive interferometric multimode fiber optic switch and modulator.xe2x80x9d It suffers from the same kinds of difficulties as Bergland. Additionally, Carlsen uses expensive specially made crystals.
Transparency is a very important consideration in the design of photonic communications equipment, and photonic signal switching matrices use in optical computers and the like. In order to provide reliable, high speed, practical switching all aspects of an incoming signal must be effectively transmitted through the switch and into the output. In order to prevent the introduction of noise, and as a result, produce unfavorable bit error rates, all phase, frequency, amplitude, spatial and polarization variations of the original input signal must be reliably transmitted through the switch and into the output.
In other words, a router must be transparent, switching an entire wavefront, not just binary data. When free space optics are used, even beam quality and profile becomes important when interconnecting a number of switches into a matrix or network. Full images with their massive amounts of spatial information also need to be switched. The prior art does not address these issues.
In view of the foregoing, it is a primary object of the present invention to provide a polarization-preserved or alternatively polarization-stabilized router for photonic signals.
It is another object of the invention to provide a phase-preserving, and phase-and-polarization-preserving router for photonic signals.
One object of the present invention is to provide means and method of transparent switching of photonic signals including all their aspects and information content as represented by a series of wavefronts along with their temporal waveforms.
Another object is to provide means and method of switching and routing images based on their temporal and/or spatial content.
It is a further object of the invention to provide a photonic switching mechanism capable of switching entire arrays of pixels maintained in a coherent pattern. Accordingly, it is an object of the invention to transmit and switch photonic images. It is yet another object of the invention to switch a parallel image or pattern of electromagnetic energy, such as light as an array of photonic signals switched in parallel as a single photonic data signal. It is also an object of the invention to provide a photonic switch for switching serialized packets of photonic data embodied in electromagnetic radiation (e.g. visible light, laser light, infrared, etc). It is a further object of the invention to provide packetized addressing integrated within a packet in order to switch a photonic signal in a photonic switch, based upon an address portion of the photonic signal itself Accordingly, it is an object of the invention to provide a switch mechanism capable of reading, in real time, an address portion of a photonic data packet, and switching a photonic data packet in accordance with the address therein.
Consistent with the foregoing objects, and in accordance with the invention as embodied and broadly described herein, an apparatus and method are disclosed, in suitable detail to enable one of ordinary skill in the art to make and use the invention. In certain embodiments an apparatus and method in accordance with the present invention may provide a versatile means and method of high speed switching of complete photonic wavefronts and waveforms, and routing them based on information contained within them. A photonic signal may have a complete wavefront made up of instantaneous spatial relations along with the time-varying nature of waveforms. This signal may be as simple as a photonic beam as would be emitted from an optical fiber, and/or directed by lenses or other optical elements having a simple binary modulated gausian cross section beam. Being transparent to the data signal, an apparatus in accordance with the present invention is also able to switch a complex photonic signal having both time and spatial-varying relationships. An example would be the series of images as in a motion picture, or a series of dynamic images common to photonic transistors. (See U.S. Pat. No. 5,093,802).
In the case of the simple telecommunications signal, the present invention maintains the spatial relationships of the beam profile so the switched output signal can be easily interfaced with down-stream components including other switches in a complex switching matrix.
Signals traversing optical fibers typically have polarization fluctuations. The present invention provides polarization insensitive transparent switching and routing even though the basic switching elements may require polarized energy. Such switching functions include addressable packet switching and other routing techniques.
In the case of more complex images, the entire spatial relationships that make up the image can be focused through the present invention as a complete image so that an image or a series of images can be automatically focused into one output location or another. Thus, the present invention can be used to switch extremely high volumes of parallel information as represented by the many pixels which make up each instantaneous image.
The basic method of the present invention for accomplishing photonic signal switching is comprised of the following steps:
1) A photonic signal is directed into a first polarization separating means to provide first and second polarized signals having complementary amplitudes and polarizations orthogonal to each other.
2) Directing the first polarized signal through a first switching means controlled by a direction control signal, to provide a first switched signal.
3) Directing said second polarized signal through a second switching means, also controlled by that same direction control signal, to provide a third switched signal, and
4) Then combining those first and third switched signals to provide a first output.
At this stage, the signal can be switched on and off. To produce the equivalent of an optical single pole double throw switch, a second switched signal from the first switching means and a fourth switched signal from the second switching means are combined to provide a second output.
The result is a switch that is substantially transparent to amplitude, frequency, phase and polarization variations of the photonic signal input. What""s more, by selecting components that are able to maintain beam quality, and spatial relationships, full images may be switched.
There are many uses for high speed information switching. The performance of such devices often depends on beam quality. In order to reduce noise, and increase throughput, all aspects of a photonic signal, or group of signals must be reproduced accurately in the outputs. By maintaining phase and spatial relationships through the switch, entire complex images may be switched even through complex networks and switching matrices.
First of all, the photonic signal input is polarization separated into two paths. Even a common polarization beam splitter will separate, not just beams, but whole images having mutually orthogonal polarizations. Since the image can be considered as being made up of pixels, then two images are produced having matching pixel pairs (one in each image). These pixel pairs will have complementary amplitudes that depend upon the polarization of the original pixel.
The first polarized signal (image and all) is then photonically switched to produce first and second switched signals. The second polarized signal (image and all) is then photonically switched to produce third and fourth switched signals. The first and third switched signals are combined to produce a first output, and the second and fourth signals are combined to produce a second output.
The key to providing true transparency, is that the delay paths from polarization separation at the input to reunification at each output must be substantially the same. The switched signals must be aligned during the combining step so that matching pixel pairs are reunited so as to reproduce the original amplitude, frequency, phase, and polarizations of the original input signal. This reunification can be accomplished using a common polarization beam splitter.
In a free-space embodiment, the various signals can be collimated, or a variety of lenses can be used in order to property direct the energy through the various components. Such transparency can be used to switch complex images, to maintain beam quality through a complex matrix of switches, and/or to switch parallel multiple beams.
Energy in these two paths will have complementary amplitude fluctuations in accordance with polarization fluctuations of each pixel of the photonic input signal, but their polarizations remain orthogonal to each other. This allows these signals to be switched using a pair of switches even though they may be polarization sensitive. The signals in each path are then switched simultaneously using any of a variety of switching means, include polarization rotating switches. This produces a pair of complementary switched signals, one from each switch, that are then combined to produce a first output. A second pair of complementary switched signals, one from each switch, are then combined to produce a second output.
The switched signal pairs are recombined having the same polarization, then any input polarization changes will be stabilized. If these signal pairs are recombined having orthogonal polarizations then the original polarization fluctuations will be preserved.
Any number of input channels, any number of output channels, and any organization of duplicate network switching means can be used. Each input is separated into its polarization components before being directed into a simultaneously controlled pair of switching configurations that produce the polarization component pairs that are then recombined to provide each output channel.
Switching means can include Kerr, Pockels, Faraday, birefringent, polymer and all optical polarization rotators or any other convenient means for rotating the polarization. Polarization separating means then provide the switched signals that are then recombined.
The present invention can be used as an optical signal router that is compatible with polarization modulation techniques. It can be made very broad band so that it can switch all the channels in an entire WDM system at the same time.
Switch control can be through various methods. One way is to decode the address part of a packet to control switching routes, thereby making a packet switching router. Generally such an address is located at the front of the packet. Decoding that address produces a direction control pulse that can then be stretched so that the switches remain directed along one route for the duration of the packet in the case of binary modulated photonic signals.
Spatial modulation addressing can also be used by sampling the input photonic signal and triggering switching action based on the parallel information in the individual images. Thus the present invention can be used to sort high speed images based on their content.
Computer controlled routers can also be constructed by controlling the switches with a computer. Full photonic control can be included by using photonically-controlled switches.
The foregoing objects and benefits of the present invention will become clearer through an examination of the drawings, description of the drawings, description of the preferred embodiment, and claims which follow.